Flexography is a printing technique that uses a flexible plate with relief called cliché, able to adapt to a number of supports or printing substrates very varied.
In this printing system, liquid inks characterized by their great drying speed are used. This high drying speed is what allows printing high volumes at low costs, compared with other printing systems.
Printers are usually rotary and the main difference between these and other printing systems is the way in which the cliché receives the ink. Generally, a rotating roller made of rubber or other materials, such as polyurethane or urethane, picks up the ink that is transferred to it by contact by another cylinder called anilox, with the intervention of a scraper that removes excess ink from the roller.
The anilox is made of chromed steel engraved mechanically or ceramic engraved by laser to have a surface with alveoli or holes of microscopic size with which it transfers a light layer of regular and uniform ink to the cliché. Subsequently, the cliché will transfer the ink to the medium to be printed.
Over time, microscopic-sized sockets or holes are covered with dry ink, which reduces the effectiveness of the roller, specifically the volume of the point, so it is necessary to periodically clean them.
Commonly, aniloxes are cleaned by three different techniques, solvent washing, soda blending and ultrasonic procedures. These have limited effectiveness.
Many inks are resistant to common solvents. Also, some solvents can not be used, due to their negative effect on the environment. In ceramic anilox, some solvents penetrate through the pores of the ceramic coating to attack the metal core of the roller so that the ceramic coating can be separated from the metal core. Cleaning with ultrasonics and soda can physically damage the ceramic itself.
As an alternative to the common anilox cleaning methods, a new method based on LASER cleaning of the anilox surface has been developed.
These devices are constituted by a mechanical structure that supports the anilox and a laser resonator. The mechanical structure rotates the anilox while the laser resonator separates and volatilizes the dry ink and debris deposited in microscopic-sized sockets or holes.
Various inventions for the cleaning and maintenance of anilox based on the laser scanning of its surface are currently known.
U.S. Pat. No. 6,354,213 describes an apparatus for cleaning an anilox roller that involves the use of a laser resonator and that comprises a first drive motor that rotates the anilox roller, a laser resonator slidably fixed in a guide projecting a laser beam; an expander of the laser beam; a lens orienting the expanded laser beam towards the surface of the anilox roll so that the slag contained in the alveoli is detached without affecting the ceramic or chromium coating; a blowing device directed towards the focal point of the laser beam that expels the loosened slag; a second drive motor that moves the laser resonator, beam expander and lens in the axial direction parallel to the longitudinal axis of the anilox roll; a bearing associated to the lens, which moves on the surface of the anilox roll, maintaining the appropriate distance so that the focal point of the laser beam hits the surface of the anilox roll.
DE4427152 describes an apparatus for cleaning anilox rollers comprising a laser resonator which, through an optical system, emits a laser beam towards a mirror that orientates it towards the anilox roller. The detached slag is removed through a suction hose.
The patent DE102011013910 describes an apparatus for cleaning anilox rolls that establishes a different operating scheme. In this case, a laser resonator emits a laser beam that is guided through optical fibers to several cleaning heads that impinge a fraction of the original beam against an area of the surface of the anilox.
The patent DE102015110877 describes an anilox roller cleaning apparatus by laser radiation, in which the beam of a laser resonator incise directly on the surface of the anilox roll, the slag being removed by a band impregnated in an adhesive element.
The cleaning capacity of these devices is much higher than that of conventional methods: washing with solvents, soda blasting and ultrasonic procedures, however, the operating time is longer, since it is necessary to make several passes or sweeps depending on the degree of anilox dirt.
The sweep speed of the laser beam is limited by the combination between the power of the resonator and the frequency of the emission. The higher the power, the greater the cleaning capacity, but at the same time the higher the temperature in the cleaning zone, which is why a limit is established from which the surface of the roller will be damaged. In the same way, more frequently, greater cleaning capacity, however, the cost of the resonator equipment increases considerably.
It would be beneficial and advisable to develop an anilox roll cleaning device that, with equal power and frequency of resonator, shortens the maneuver time.
Another problem in roll anilox laser cleaning systems is the adaptation of the focal length of beam to the diameter of the roll anilox to match the focal point on the surface of the cylinder. This adaptation is done in two ways. The first, manually by means of micrometric axes that allow to radially move the focal point of the laser beam with respect to the surface of the anilox roller. This system has the disadvantages of manual mechanical adjustments, derived from the wear of parts, misalignments by vibrations, etc.
The second way of adjustment is assisted, for which the device incorporates an electronic system in which the characteristics of the anilox roll are introduced through a user interface, so that a software program determines the appropriate coordinates of the focal point of the beam and drives a servomotor that moves it radially to the calculated position. This system has the disadvantage of the possibility of error in the data entry, which implies placing the focal point in wrong coordinates and consequently the low or null operability of the laser scan.
It would be beneficial to incorporate means of automatic adjustment of the focal point without intervention of the operator.
Another problem in anilox laser cleaning systems consists in the lack of means to determine if the anilox roller is rotating properly in its support bed. There have been cases in which, due to wear of the tractors of the roller, by jamming the axis of rotation, due to lack of alignment or irregularities in the surface of the anilox, this can rotate irregularly or even stop, thereby an overexposure of the surface of the anilox to the laser beam occurs, being irretrievably damaged.
It would be beneficial to incorporate security means that would stop the laser scan if the rotation of the anilox roller is irregular or stopped accidentally.